With the ongoing climate changes and raise of population, the demands of crop yield are continuously increasing (International Panel on Climate Change, 2007; Tilman et al, 2011). In addition to minimizing CO2-emissions from agriculture, developing crops that cope better with suboptimal growth conditions, like drought, salinity and increasing temperatures is necessary. Manipulations or control of the development of—and signaling in plants are ways to improve the tolerance responses in plants and to ensure their flexibility in a changing environment without negative effects on yield. Although treatments of plants with exogenous plant hormones such as abscisic acid (ABA) have been shown to improve stress tolerance, the use of ABA by topical spraying, to protect plants against the effects of drought conditions is limited because of pleiotropic side-effects, such as an inhibited primary root growth (Sreenivasulu et al, 2012). Moreover, ABA is light-sensitive and degrades rapidly when in contact with plants (Hao et al, 2010). These drawbacks stimulated the screening for new molecules that switch on the ABA-signaling pathway more specifically, without the side-effects and with better stability to make applications in agriculture feasible (Joshi-Saha et al, 2011; Melcher et al, 2009).
In 2009, a chemical genetics screen resulted in the identification of an ABA-agonist: pyrabactin, which is structurally not related to ABA (Park et al, 2009). Microarray analysis of ABA-responses in seeds and seedlings suggested that pyrabactin is a selective agonist of ABA (Kitahata & Asami, 2011; Park et al, 2009). The further screening of pyrabactin-resistant mutants made it possible to identify a family of ABA-receptor proteins: the PYR1/PYL/RCAR START proteins (Kitahata & Asami, 2011; Park et al, 2009). Although there are no apparent chemical or structural similarities between the sulphonamide pyrabactin and ABA, they both act through the PYR/PYL receptor family (Hao et al, 2010). However, pyrabactin does not provoke all the responses described for ABA (Park et al, 2009; Peterson et al, 2010). Furthermore, also the interaction with the receptors differs between the two molecules. Pyrabactin acts as an antagonist of PYL2, which means that pyrabactin-binding in the receptor does not stimulate the attraction of PP2Cs and thereby does not remove the inhibitory factors for downstream signaling. This is in contrary to ABA, which is an agonist of this receptor (Melcher et al, 2010a; Mosquna et al, 2011). This observation suggests that pyrabactin differentially modulates ABA-receptors activity and may even target only a subgroup of receptors. However, since its discovery, pyrabactin has not been applied to improve crop production in for example drought conditions. Sulphonamides have indeed been described to have negative effects on plant growth and are even used as herbicides (Audus & Quastel, 1948; Crowdy & Jones, 1956). Moreover, the effects of pyrabactin in vegetative tissue are limited and the ABA-agonist activity is specific to seeds.
Alternative compounds to pyrabactin with improved chemical properties are thus highly needed.